Even though the Epic Link should not be editable from the Field Configuration page the customer could benefit if we have the ability to make it required to ensure all the issues created have a parent Epic. It would be similar to the example below:

Workaround

Documented, here.

While a 'Locked field' can't be set as Mandatory in the field configuration, there is an alternative to add a Validator in the workflow. Be aware that the following changes will be reflected in all projects that share the workflow. Also, you must make sure that the Epic Link field has been add to your create screen. If it is not there and you make it required, users will not be able to create issues.

1. Go to your company-managed project, Project settings > Workflows
2. Click the pencil icon to edit the workflow
3. Click on the Diagram tab and then click on the 'Create' transition and click Validators from the pop-up screen:
   
4. Click 'Add validator' and then select "Field Required Validator"
5. Choose "Epic Link" for "Fields required for this transition"
6. Enter an error message which will be visible to the user. I recommend including "Workflow validation" in the error message so the source of the error is clear to your team.
7. Click Add
8. Save the workflow

Here is how the Create screen looks if I don't fill out the epic link field:

Note: There is no red asterisk next to the Epic Link field, because it is required by a workflow validator (rather than being made required in a Field Configuration). There is a feature request to add such UI highlight, here: JRACLOUD-76479 – Make validator required fields more visible on screens